This invention relates to an improvement in a rotary fluid machine, of the single screw, gate rotor type which may be employed as a compressor, a motor or a pump.
Our prime interest is with regard to signle screw, multi-gate rotor machines when used as compressors (e.g. for compressing air or a refrigerant vapour or gas) and for simplicity, this specification thereafter refers to the mode of use in which compressible fluid is fed to the machine through a low pressure inlet port and is exhausted from the machine at a higher pressure through an outlet port. It should be appreciated, however, that it is believed that the invention applies equally to alternative modes of operation in which the machine is used to generate kinetic energy from a fluid supplied at high pressure (i.e. operation as a motor).
This invention is specifically concerned with rotary fluid machines of the kind comprising a screw rotatable about an axis and having surface grooves formed therein which are inclined relative to that axis, the lands, serving to separate the grooves one from another, making sealing engagement with a surrounding casing whereby each groove defines a chamber with the casing, during at least a part of the rotation of the screw within the casing, a gate rotor having teeth which intermesh with the grooves of the screw, each tooth being successively in sealing relationship with the grooves as the intermeshing screw and rotor rotate, the volume of any chamber defined by a groove and limited at one end by a rotor tooth changing from a maximum to a minimum as the screw and rotor rotate, a high pressure port in the casing adjacent to a high pressure end of the screw and communicating with each chamber when the latter is at, or adjacent to, its minimum volume and a low pressure port at a low pressure end of the screw and communicating with each chamber. Hereafter, throughout this specification a rotary fluid machine of the kind just described, will be referred to as a "rotary fluid machine of the kind specified".
Typically a rotary fluid machine of the kind specified would have two gate rotors disposed diametrically with respect to the screw, there being low and high pressure ports associated with each gate rotor.
When a rotary fluid machine of the kind specified is used as a compressor, fluid to be compressed is supplied through the low pressure port. The geometry of the intermeshing screw and rotor(s) together with the size of the high pressure port(s), would be selected to give a desired volume ratio (i.e. ratio between the volume of the chamber when filled with fluid at the pressure existing in the low pressure port and when communication with that port has just ceased, to the volume of the chamber when that chamber first communicates with the high pressure port) but in many applications it is desirable to be able to modify the capacity of the machine (i.e. to modify the volume of gas compressed to the desired volume ratio per unit time) without altering (to any appreciable extent) the speed of rotation of the intermeshing screw/rotor(s) and without seriously modifying the designed volume ratio.
If the volume ratio is allowed to fall and the machine is working across a fixed pressure difference, the compression becomes inefficient resulting in reduced efficiency at part load. A rise in volume ratio is even less desirable because in addition to the power lost in over-compressing the gas, the higher pressures occurring give rise to corresponding higher leakage losses.
To this end, it has been proposed in the specification of U.S. Pat. No. 4,074,957 of Clarke et al (hereafter referred to as the former specification) to provide a capacity-regulating valve in the casing adjacent to the high pressure side of the or each gate rotor, said or each said valve including a channel which communicates with the grooves and extends beyond the high pressure end of the screw, the channel being provided with a movable capacity-regulating member which in one limiting position obturates the one end of said channel which is remote from the high pressure end of the screw while leaving a region of said channel open adjacent said high pressure end and in the other limiting position extends beyond the high pressure end of the screw and leaves open the channel at the said one end. Suitably the said one end of the channel is located at a point intermediate the low and high pressure ends of the screw.
Using a capacity-regulating valve as described in the former specification, it is possible to provide a machine having a facility for modifying the capacity continuously from 100% to 25% with a reduced variation in the volume ratio occurring throughout that adjustment range.
However, when using the arrangement described in the former specification, even the reduced variation in volume ratio may be excessive and this invention relates to an improved arrangement which in preferred embodiments enables large capacity reductions to be effected with further reduced variations in volume ratio.